Posts for Tag: avr

Some AVR memory questions

I had a few Arduino itches I wanted to scratch, bits and pieces which aren't particularly complex or interesting enough to warrant a standalone blog post, but which are nonetheless worth spending a few paragrapsh on.

Why does every Arduino build include "-R .eeprom" when producing the ELF binary?

I had previously wondered whether it was necessary to pass in "-R .eeprom" when building the ELF binary, but I didn't really explore it much at the time. Essentially all articles or guides about using the GNU toolchain to compile programs for Arduino/AVR (and indeed the Arduino IDE itself) specify this switch so it's really worth clarifying what it's all about.

The first thing is that some articles are just plain wrong in their documentation about this option, for example the Arduino Playground docs for FreeBSD state that it's required to upload our binary to the Arduino board:

This is pretty wrong, since in avr-objcopy the -R switch actually specifies sections to be removed from the binary - per the manpage below:

So let's investigate what's up - here's an avr-objdump on the binary produced by blink.c:

There's actually no .eeprom section here to be removed - so nothing in binary would end up there anyway. If we define a variable with the necessary attribute "__section__" we can force something to be placed here, like the following string:

char *foo __attribute__((__section__(".eeprom"))) = "bar";

and now if we rebuild our ELF binary and inspect using avr-objdump:

All of a sudden we've got an .eeprom section in our executable, which would be removed if we followed the default instructions.

So why is this removed by default when uploading an Arduino sketch? I can only guess, but presumably it's just a common courtesy to anyone who uses .eeprom as persistent runtime storage - so they don't accidentally hose their data when programming their board. Hopefully if you know enough to place a variable in .eeprom, then you'd have the know-how to be able to alter the build configuration to omit these flags. It could also be possible that the eeprom wears out after fewer writes than internal flash so a bit of extra care is taken to preserve it - though I really cannot say why.

How is Arduino memory laid out and what controls it?

I was curious about how the memory layout is defined in the avr-gcc toolchain used in building programs for the Arduino, and ow it can be controlled. I've played around with LDF files for Analog Devices' Blackfin and SHARC chips (which look like ADSP-BF533.ldf and ADSP-21160.ldf respectively), but have never used GCC's linker scripts before.

On OS X using CrossPack these linker scripts reside in /usr/local/CrossPack-AVR/avr/lib/ldscripts, and there are an astonishing 95 of them in total:

These linker scripts aren't defined per microprocessor (i.e. atmega328, atmega168), but are written per architecture - taking a quick look at the documentation at nongnu.org we can see that the AVR chips used in current Arduino boards are all in the avr5 family (except for the atmega2560, which is avr6) so we can narrow things down slightly, but there are still five linker scripts to choose from:

Which one of these is used depends on the arguments passed to the linker, and each of them should be different and tailored for a certain purpose - although in practice this is not strictly true:

  • avr5.x: default linker script, this is used for linking straight C programs
  • avr5.xn: linker script used when the "-n" flag is passed to the linker. According the the ld manpage this flag is used to turn off page alignment and mark the output as "NMAGIC" however diffing this and avr5.x shows no difference in practice.
  • avr5.xbn: used when the "-N" flag is passed to ld. It's meant to mark the .text and .data sections to be readable & writeable (as well as not page-aligning the data segment apparently) however again this is script is identical to avr5.x.
  • avr5.xr: used when "-r" flag passed to ld. This is to "generate relocatable output" - but essentially it's used to pre-link C++ files I think?
  • avr5.xu: used when "-Ur" is passed to ld. This is used for the final link on C++. It's roughly the same as avr5.xr but includes something about ctor table

To see what the memory layout the basic linker script produces, we can take a pretty boring program like blink.c and take it apart using avr-objdump. The GCC docs have a nice wee visualisation of the SRAM usage of an atmega128, which won't be a million miles away from the 328p:

So the SRAM contains Data, BSS (a segment containing variables initialised to zero) and then some space for the heap (which starts at the address __heap_start and continues to a handful of bytes before the stack base) and the stack begins at RAMEND (address 0x8FF on Atmega328p) and grows down as necessary.

This is why you get that little "low memory" warning in the Arduino IDE - if there's only a couple hundred bytes spare in SRAM it's not too hard to accidentally write a program that uses too much stack and ends up overwriting some of heap variables. There's no memory protection (that I'm aware of) on these chips so there's no way to prevent this other than being careful.

How quickly can we access the Arduino's SRAM, Program Memory and EEPROM?

I wrote a couple of simple benchmarks to see how quickly we can access the different memory types available on the Arduino:

  • read: read and sum a 256 byte array
  • write: write the values 0...255 into a 256 byte array

This is not massively scientific but I just wanted ballpark figures to see how things stacked up. The results I got were:

memory read (μs)
write (μs)
data 148 148
program 196 n/a
eeprom 584 859580

I was pretty surprised by how fast reads from program memory could be, and even more so how quick eeprom reads were - but slow eeprom writes aren't particularly surprising. It's worth noting that for program memory at runtime you can only write to certain sections reserved for the bootloader and I want to risk messing that up so I left it out.

A tiny new font for the SSD1306 128x64 OLED screen

The default font for the Adafruit graphics library is a 5x7 font, it looks a little something like this:

It's readable, but I thought it'd be interesting to use a more compact font like "Tom Thumb" by Robey Pointer:

The font currently bundled with the library is in a file called glcdfont.c and is defined as a big byte array:

Each line of that array is 5 bytes and represents a simple monochrome bitmap of a single ascii character, with each byte representing a column. For example example the letter "a" is represented by the following line:

0x20, 0x54, 0x54, 0x78, 0x40,

In binary each of these is:

You can almost see the shape if you twist your head 90 degrees to the right, but to make things a little clearer here's what this actually represents:

If we were to use the Tom Thumb font we could save ourselves two bytes per character, since it is a little slimmer, meaning that "a" could be represented by the following three bytes:

0x68, 0x58, 0x70

Or, in binary:

Which looks like this:

Since there's a bit of wasted space at the top (the top two bits won't get used at all) we could technically save ourselves a little more space, but for simplicity's sake I'm going to sacrifice those two bits. Currently the font can be easily retrieved and manipulated as each column fits neatly into a single byte, however we'd introduce a good deal more complexity by squeezing those additional bits - which would mean substantial changes to the driver itself to compute the correct address to read the bitmap from, and shift/mask the necessary bits.

However even with this tradeoff we can still save 2 bytes per character, which is a pretty impressive 512 bytes over the whole ascii space which is pretty substantial.

The main challenge is creating a separate .c file we can swap glcdfont.c out for. I had trouble dealing with the BDF file Robey shared, but since there's a little bitmap representation of the whole ascii space it wasn't too tough recreating the font by hand ... just extremely tedious! I hacked together a simple little app (source is here) using Processing that let me paint each character by clicking boxes and which spat out the hex values when I hit a key. I'd then copy and paste this into the glcdfont.c. As I said, extremely tedious.

Once I'd produce my file it was time to test it - the beauty of having the 3x6 font means that the entire ascii space could be displayed on a 128x64 screen:

It's actually pretty surprising how much text you can fit on screen.

And just a little reminder of how small this screen is, with a €2 coin for scale

You can see the amount of memory saved in the .text section (where program code lives) by examining binaries GCC has produced - exactly 512 bytes.

My fork of the Adafruit gfx library with the Tom Thumb font is at https://github.com/smcl/Adafruit-GFX-Library and is a drop-in replacement for the existing library (replace the entire library, though, since I made some changes in Adafruit_GFX.cpp. You can switch between the fonts by toggling the #if ... #else condition in glcdfont.h below:

Apes and Embedded Systems (part two)

In part one I just hooked up a little monkey keychain called Chuggzilla to my Arduino. A day or so prior to my 90 Kč spending spree on Chuggzilla I'd just received a 128x64 Adafruit OLED display and realised this was a nice opportunity to play a bit with it.

The idea was that Chuggzilla would still scream his weird morse code madness, but the screen would display the actual text behind the message, with the current letter highlighted. I was just wanting to quickly get something working, so instead of integrating with the OLED library using C I just ported my morse code stuff to the Arduino IDE so I could easily re-use the existing API. The final result is pretty fun - though I suspect the current drain from the screen is too high because the previously manic ape noises were almost whisper quiet.

I am not sure exactly who would want to reproduce this, but if you're interested hook the OLED screen up as per the Adafruit wiring instructions then connect up whatever animal or thing you want to control per my original schematic (connecting digital 2 instead of digital13 to the transistor's base, since we need that for the screen), create a sketch with the code below and add the Adafruit libraries.

Code is on this gist on github. 

Apes and Embedded Systems (part one)

I woke up today and decided to do some neurosurgery. I set off into the town and sought out a willing test subject - ladies and gentlemen, meet Chuggzilla (credit goes to Miriam for the name):

Chuggzilla is a keychain monkey I bought in Tiger for around 90 Kč. Pressing a little button on the back of his head causes his LED eyes to light up and a little speaker inside to play a monkey noise:

This is what he looks like on the inside

I soldered a couple of wires to either side of the button element, so that I could control him electronically:

Testing him out with a button on a breadboard

Finally being controlled (via a transistor) by an output pin on the Arduino using the morse code program I previously wrote - the message is "hello world" or ".... . .-.. .-.. --- / .-- --- .-. .-.. -.."

The circuit driving this is below. I used a little LED for debugging when I was checking Chuggzilla was connected fine, and just kept it in the circuit - you can omit it if you're certain your animal is wired up fine :)

The code behind this is at https://github.com/smcl/arduino_morse - it's straight C, so if you want to load it to your Arduino you'll need to have the AVR GCC tools installed, and change the AVRDUDE_PROGRAMMING_DEVICE variable in the Makefile, then run make flash.

Arduino primer/refresher - OS X perspective

I've been wanting to get back into some interesting embedded systems and electronics work - so I figured I'd dust off my old Arduino. In the process I ended up having to re-learn how to program it using C without using the Arduino IDE (it's not my favourite thing), which is what I'm going to go into here.

I downloaded CrossPack which contains GCC cross compiler for the AVR architecture, as well as a handful of other useful tools (like avrdude, which we'll use to flash the Arduino). Once that's setup we can start off with a pretty simple program which rapidly blinks the arduino's onboard LED, which is sort of the Hello World of the Arduino community:


To build this using GCC we just need to call avr-gcc, making sure to specify the processor (using -mmcu), define the processor frequency (the F_CPU macro)


This creates an ELF binary. One odd thing is that pretty much everyone building C code for Arduino seems to include the "-R .eeprom" switch which removes the ".eeprom" section from the output binary. However I've not ever seen this section does not exist in my ELF binary - see below:


Anyway, perhaps there's something else at play that I've not understood so I've kept this switch in (I'm making a mental note to explore this later). Again many other places seem to insist on using the avr-objcopy utility to produce intel hex file before uploading to the Arduino - so often you'll see the below:


However again this is not a necessary step it seems, since the utility we use to upload the binary to the Arduino - avrdude - supports ELF so you can skip it. When we want to upload the binary to the Arduino we'll use avrdude, but first we need to know the programming device. For me this is just the USB type A to type B cable that comes with the arduino, which in the example below is /dev/tty.usbmodem1421:


I've found it useful to create a little Makefile to tie these steps all together:


Finally to tie this together in a slightly more existing example than blink.c, I've created a little program that will flash the LED attached to pin 13 with a message encoded in morse - "hello world" or ".... . .-.. .-.. --- / .-- --- .-. .-.. -..":


The code is on github at https://github.com/smcl/arduino_morse and only requires a bare Arduino and USB cable.